Demand for rechargeable lithium secondary batteries as power sources of portable electronic devices for digital communication, such as personal digital assistants (PDAs), cellular phones and notebooks, as well as electric bicycles and electric automobiles is rapidly growing. The performance of these devices is greatly influenced by secondary batteries as key components of the devices. Accordingly, there is a strong need for high-performance batteries. Several main characteristics required in batteries are charge-discharge characteristics, cycle characteristics, high-rate characteristics, and high-temperature stability. Lithium secondary batteries have drawn attention because of their high voltage and high energy density.
Lithium secondary batteries are classified into lithium metal batteries using lithium as an anode and lithium ion batteries using an interlayer compound, e.g., carbon, capable of intercalation/deintercalation of lithium ions. Further, lithium secondary batteries are classified into liquid type batteries using a liquid electrolyte, gel type polymer batteries using a mixture of liquid and polymer electrolytes, and solid type polymer batteries using a pure polymer electrolyte according to the kind of electrolytes used.
Currently available small-size lithium ion secondary batteries use LiCoO2 as a cathode material and carbon as an anode material. Lithium ion secondary batteries using LiMn2O4 as a cathode material were developed by Moli Energy Corp., Japan, but their use is negligible in comparison with that of batteries using LiCoO2. Extensive research on LiNiO2, LiCoxNi1−xO2 and LiMn2O4 is actively underway as cathode materials. LiCoO2 is a promising material in terms of its stable charge-discharge characteristics, high electronic conductivity, superior thermal stability and constant discharge voltage characteristics, but has disadvantages of insufficient cobalt deposits, high price and toxicity. For these reasons, development of novel cathode materials is needed. LiNiO2 has problems that it is difficult to synthesize and is thermally unstable, which make LiNiO2 unsuitable for commercialization. Commercial applications of LiMn2O4 are currently limited to some low-priced products. However, LiMn2O4 is has a spinel structure and deliverers a lower theoretical capacity (˜148 mAh/g) than other active materials. In addition, LiMn2O4 has poor cycle characteristics due to the Mn dissolution into electrolyte. Particularly, since LiMn2O4 has poor high-temperature characteristics at 55° C. or higher when compared to LiCoO2, it has not yet been put to practical use in batteries.
To overcome these problems, numerous studies have focused on materials having a layered crystal structure. Under such circumstances, Li[Ni1/2Mn1/2]O2 (nickel:manganese=1:1) and Li[Ni1/3Co1/3Mn1/3]O2 (nickel:cobalt:manganese=1:1:1), each of which has a layered crystal structure, are currently in the spotlight. These materials are advantageous over LiCoO2 in terms their low price, high capacity and superior thermal stability.
However, since Li[Ni1/2Mn1/2]O2 and Li[Ni1/3Co1/3Mn1/3]O2 have a lower electronic conductivity than LiCoO2, they show poor high-rate characteristics and poor low-temperature characteristics. Further, since Li[Ni1/2Mn1/2]O2 and Li[Ni1/3Co1/3Mn1/3]O2 have a low tap density, no improvement in the energy density of batteries is achieved despite their high capacity. Particularly, the electronic conductivity of Li[Ni1/2Mn1/2]O2 is extremely low, which causes difficulty in the commercialization of the material (J. of Power Sources, 112 (2002) 41-48). Particularly, Li[Ni1/2Mn1/2]O2 and Li[Ni1/3Co1/3Mn1/3]O2 show poor high-power characteristics over LiCoO2 and LiMn2O4, which makes them unsuitable as materials for hybrid power sources for use in electric automobiles. In an attempt to solve such problems, Japanese Patent Laid-open No. 2003-59491 suggests a method for treating the surface of a cathode active material with conductive carbon black. However, significant improvement has not hitherto been reported.
Lithium secondary batteries have problems in that the cycle life is drastically shortened due to repeated charge-discharge cycles, especially at high temperatures. The reason for this is that electrolytes are decomposed, active materials are degraded, and the internal resistance of batteries is increased due to the presence of moisture within batteries and other factors. A number of efforts to solve these problems have been made. For example, Korean Patent No. 10-277796 discloses a cathode active material surface-coated with a metal oxide, such as an oxide of Mg, Al, Co, K, Na or Ca, by annealing. A technique for improving the energy density and high-rate characteristics of lithium secondary batteries by adding TiO2 to an active material, e.g., LiCoO2, is suggested (Electrochemical and Solid-State Letters, 4 (6) A65-A67 2001). A technique for prolonging the cycle life of lithium secondary batteries by treating the surface of natural graphite with aluminum is known (Electrochemical and Solid-State Letters, 4 (8) A109-A112 2001). However, the problems of the shortened cycle life and gas evolution arising from the decomposition of electrolytes during charge and discharge still remain unsolved. Further, active materials may be dissolved by acids formed from the oxidation of electrolytes during charge due to a reduction in the capacity of batteries (Journal of Electrochemical Society, 143 (1996) P2204). In recent years, Korean Patent Laid-open No. 2003-0032363 describes a technique for coating the surface of a cathode active material with a hydroxide, oxyhydroxide, oxycarbonate or hydroxycarbonate of Mg, Al, Co, K, Na, Ca, Si, Ti, Sn, V, Ge, Ga, B, As, or Zr.